1. Field of the Invention
The present invention relates to a method of softening rolled medium carbon machine structural steels, particularly those which are to be worked into bolts, nuts, shafts and other shapes by cold forging.
2. Prior Art
Prior to the production of machine parts from medium carbon machine structural steels by cold forging, the steels are customarily subjected to cementite spheroidization annealing with a view to softening them, or reducing their resistance to deformation. This softening treatment takes as long as 10-20 hours and it has long been desired to develop a soft rolled steel that needs no spheroidization annealing, thereby achieving improved productivity or reduced energy consumption.
While various proposals have been made in an attempt at attaining this object, "Tetsu to Hagane (Iron and Steel)", 70, 5, 236, 1984 proposes that the medium carbon machine structural steels specified in the currently effective JIS (e.g. S45C and SCM435) should be softened by rolling at low temperatures near 675.degree. C. and by subsequently holding them at a specified temperature. This method, however, is not considered a satisfactory solution because rolling in the low temperature range will cause surface defects in wires or reduce the durability of working rolls.
There exists much patent literature proposing techniques for the need to eliminate spheroidization annealing. Laid-Open Japanese Patent Publication No. 107416/1983 shows a softening method wherein a steel is roughing-rolled to achieve a reduction in thickness of 30% or more at temperatures not lower than 1,000.degree. C., then finish-rolled to achieve further reduction in thickness of 50% or more in the temperature range of 750.degree.-1,000.degree. C. and, thereafter, is cooled to the end point of transformation at a cooling rate not faster than 1.degree. C./sec. Laid-Open Japanese Patent Publication No. 13024/1984 shows a carbide spheroidization technique wherein a steel is finish-rolled to achieve a reduction in thickness of 30% or more in a temperature within the limits of a value not higher than the Ar.sub.1 point and one not lower than the point of Ar.sub.1 minus 50.degree. C., and the rolled steel is reheated in the temperature range of Ac.sub.1 -Ac.sub.3. Laid-Open Japanese Patent Publication No. 126720/1984 discloses a carbide spheroidization technique wherein a steel is finish-rolled to achieve a reduction in thickness of 80% or more in a temperature range within the limits of a value not higher than the Ar.sub.1 point and one not lower than the point of Ar.sub.1 minus 50.degree. C., and the rolling operation then is finished at a temperature in the range of Ac.sub.1 -Ac.sub.3 by using the heat resulting from rolling. In the method shown in Laid-Open Japanese Patent Publication No. 126721/1984, the rolled steel is immediately cooled to produce a spheroidized carbide. Laid-Open Japanese Patent Publication No. 136421/1984 proposes a carbide spheroidization technique wherein a steel is finish-rolled to achieve a reduction in thickness of 10% or more in a temperature range within the limits of a value not higher than Ar.sub.1 and one not lower than the point of Ar.sub.1 minus 200.degree. C., the rolled steel is heated to a temperature in the range defined by a value not higher than the Ac.sub.3 point and one not lower than the point of Ac.sub.1 minus 100.degree. C. using the heat resulting from rolling, and the steel then is cooled from that temperature to 500.degree. C. at a cooling rate not faster than 100.degree. C./sec. In the method disclosed in Laid-Open Japanese Patent Publication No. 136422/1984, the heated steel is held for 7 minutes or longer in the temperature range defined by a value not higher than the Ae.sub.1 point and one not lower than 500.degree. C., so as to produce a spheroidized carbide. The method shown in Laid-Open Japanese Patent Publication No. 136423/1984 attains the same object by subjecting the steel to repeated cycles of controlled rolling wherein the steel being rolled is cooled to a temperature not higher than the Ar.sub.1 point but not lower than the point of Ar.sub.1 minus 200.degree. C., subsequently rolled to achieve a reduction in thickness of 15% or more, and heated to a temperature not lower than the Ac.sub.1 point but not higher than the Ac.sub.3 point by using the heat of deformation. Each of these techniques, however, involves the problems of increased surface defects and reduced durability of working rolls since, in comparison with ordinary hot rolling which is finished at about 1,000.degree. C., these techniques have to attain great decreases in thickness at lower temperatures.
As is well known (see, for example, Laid-Open Japanese Patent Publication No. 136421/1984 mentioned above), rolled medium carbon steels usually have either the pearlite or ferrite-pearlite structure. Therefore, in order to reduce the strength of rolled medium carbon steels, it is necessary to reduce the strength of the pearlite that accounts for the greater part of the structure. In view of the generally established theory that the strength of pearlite is inversely proportional to the interlamellar spacing of the cementite in the pearlite, the interlamellar spacing must be increased if one wants to decrease the pearlitic strength.
However, the interlamellar spacing of cementite in the pearlite is uniquely determined by the temperature at which pearlite transformation occurs from austenite, and the higher the transformation point, the more coarse the interlamellar spacing of the cementite. This means that in order to soften a rolled medium carbon steel, transformation to pearlite must be occurred at high temperatures by either cooling the as-rolled steel slowly or by immediately holding the as-rolled steel at the highest possible temperature in the range wherein such pearlite transformation takes place. However, the rate at which the pearlite transformation proceeds decreases with increasing temperatures and an excessively long period is required before the transformation is completed if it is transformed at higher temperatures. The problem is that whichever of the two softening methods is employed, the equipment or production line available today has inherent limitation with regard to the rate of slow cooling or the period for which the rolled steel is maintained at the highest temperature that is practically possible.